Effect of Loading Rate on the Impact Fracture Toughness of Acetal and Polymethyl Methacrylate

This investigation is concerned with evaluating the suitability of fracture mechanics testing and analysis methods for characterizing the impact fracture resistance of polymers. The polymers characterized in this study were acetal and polymethyl methacrylate. A precracked three-point bend specimen configuration was selected for fracture characterizations, with tests being conducted at rates from 2.12 × 10−5 to 1.06 m/s. Tests at each loading rate were conducted with specimens having three different crack-length-to-specimen-width ratios, a/W. Specimens were instrumented with a clip gage at the notch opening, and a potential drop technique was also employed during testing to delineate critical crack instability. All the tests were performed on an Instron servohydraulic instrumented impact test system specifically designed and instrumented for impact testing of polymers. Plane-strain fractures were observed for all the specimens tested, and linear elastic fracture mechanics methods were employed for data analyses. Plane-strain fracture toughness was observed to be relatively constant with increasing a/W ratio for both polymers, except for those specimens tested at the highest loading rate. It was also found that fracture toughness was significantly influenced by loading rate, with transitions in toughness occurring for both polymers at particular rates of loading. The authors postulate that these transitions are associated with molecular relaxation mechanisms and a change of thermal state at the crack tip.